Haskell嵌套代数数据类型

我试图在Haskell中对以下简单的Scala ADT进行建模:

sealed trait Value

sealed trait Literal < Value
case object Null extends Literal
case class IntLiteral(value: Int) extends Literal

case class Variable(name: String) < Value

我为 Literal

Prelude> data Literal = Null | IntLiteral Int deriving (Show, Eq)

Prelude> Null
Null
Prelude> Null == IntLiteral 3
False
Prelude> IntLiteral 3 == IntLiteral 3
True

现在我试着介绍一下 Variable :

data Value = Literal | Variable String deriving (Show, Eq)

Prelude> Null == Variable "foo"

<interactive>:3:9: error:
    • Couldn't match expected type ‘Literal’ with actual type ‘Value’
    • In the second argument of ‘(==)’, namely ‘Variable "foo"’
      In the expression: Null == Variable "foo"
      In an equation for ‘it’: it = Null == Variable "foo"